A second condition produces expression of incredulity (2,2)
I believe the answer is:
as if
'expression of incredulity' is the definition.
(as if!)
'a second condition' is the wordplay.
'second' becomes 's' (common abbreviation).
'condition' becomes 'if' (as in 'a big if').
'a'+'s'+'if'='AS-IF'
'produces' is the link.
(Other definitions for as if that I've seen before include "highly unlikely" , "Or though" , "Yeah right!" , "Seemingly" , "a voice of doubt" .)